The present invention relates to a method and apparatus for the joining of the edges of webs or sheets comprising heat-sealable material.
In packaging technique, for example, containers are frequently manufactured in such a manner that a sheet or a web of packing material is folded to a tube by joining together the longitudinal edges of the sheet or web, whereupon the tube formed is used in various manners as a container body or as a strengthening casing.
It is known that tubes of the type referred to here can be formed by using sheet material which is heat-shrinkable. Such a shrinkable material can be obtained if e.g. an extruded foamed plastic film is stretched in connection with the manufacture, as a result of which the film obtains a cellular structure with oblong, extended cells. Such a "stretched" foamed plastic material, e.g. polyestyrene foamed plastics, will shrink on being heated to the softening temperature of the plastics since the cells then return to their normal sperical shape. A foamed plastic material of the type referred to here has found application for the shrink forming of cups and the like by placing a cylinder or casing of the material around a profiled mandrel and heating the material to shrinking temperature. In this way the cylinder or casing shrinks and assumes the shape of the mandrel. After cooling, the shrunk casing, which then has obtained cup-shape, can be removed from the mandrel. As mentioned previously, a cylindrical casing is used as a starting object in the shrink forming of cups or other container bodies, and this casing in turn has been produced from a plane sheet or web of shrinkable material.
If the material consists exclusively of foamed plastics, the mechanical properties will be poor, since the foamed plastic material has relatively poor intrinsic rigidity and a very low tensile strength. A much better material from a point of view of tensile strength can be obtained if the foamed plastic material is laminated with homogeneous plastic layers. It is known that such laminates can be used in connection with shrink forming, since the outer homogeneous plastic layers will be shaped together with the foamed plastic layer during this shrinkage.
Such a laminated material may consist of a central layer of polystyrene foam plastics and outer homogeneous layers of polystyrene. Since the outer layers, which are relatively rigid, are placed at a relatively great distance in relation to one another (approx. 1 mm or more) through being separated by the central layer of foamed plastics, the laminate will have very great rigidity in relation to its weight. The tensile strength properties too will be considerably improved since the homogeneous plastic layers have a substantially higher tensile strength than the foamed plastic layer.
If a sheet of foamed plastics or of the abovementioned laminate is formed to a tube by making the edge zones of the sheet overlap one another, the joint zone will have a considerable thickness, since the laminate comprising foamed plastic layers is in most cases substantially thicker than corresponding materials e.g. paper or homogeneous plastic material. This thick joint zone is for one thing unsightly and for another the joint zone may pose problems of tightness in connection with the attachment of the top and bottom parts, since the joint constitutes a considerable discontinuity arourd the periphery of the casing or tube. Furthermore, in an overlap joint of the type mentioned, only the laminate layers facing one another are joined together. The inner and outer layers in the overlap joint as well as the central foamed plastic layer are not included in the joint.
These disadvantages are overcome by the present invention, which is characterized in that one or more webs or sheets are combined along their edge zones with simultaneous movement in longitudinal direction of the edge zones. The edge zone of the one web or sheet is guided, prior to the combination, past a cutter device, by means of which the central portion of the edge zone is cut away. Then one of the edge lugs formed during the cutting is folded away and the edge zone of the unmachined web or sheet is fitted into the space of the machined edge of the web which has been cut out. The edge lug that has been folded away is returned to its original position, and heat is supplied to the joint zone comprising the two combined edge zones which are compressed with the surfaces melting together, so that a tight and mechanically durable joint is obtained.